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Twin gene idea could wipe out malaria mosquito

By James Randerson

Every 10 seconds a child dies from malaria – but there may now be a way to control the Anopheles mosquito that spreads the disease.

Scientists say they can genetically modify whole mosquito populations so that the flies are either susceptible to pesticides, or can’t transmit the malaria parasite. And they would only need to release relatively few GM mosquitoes to kick-start the process.

According to the WHO, malaria claims more lives than any other communicable disease except tuberculosis, infecting around 500 million people each year. Yet it’s proved impossible to eradicate the mosquitoes that carry the disease, because the insects are so numerous.

One trick that has worked against the tsetse fly in Zanzibar and the screw worm fly in North America is to release billions of sterilised males to swamp the wild population. Almost all the wild females mate with the sterilised males and the population crashes.

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But it’s extremely difficult to rear huge numbers of sterile male Anopheles mosquitoes in captivity as females require constant blood meals to lay eggs, says Chris Curtis at the London School of Hygiene and Tropical Medicine. And anyway, it’s only a short-term solution – even if you could wipe out all the mosquitoes in one area, nearby populations would soon take over.

Jumping genes

Another idea is to modify a population so that it no longer carries the parasite. Two potential methods involve introducing a GM bacterium or harnessing so-called “jumping genes” that occur naturally in fruit flies. But there are fears that the bacteria could spread to other species. And the fruit fly genes won’t spread particularly well through populations of Anopheles.

Now Stephen Davis’s team at the Commonwealth Science and Industry Research Organisation in Canberra, Australia, think they have found a way to safely “infect” whole populations of mosquitoes with detrimental genes.

Davis’s idea is to release engineered males that have two copies of a “type A” gene, and two copies of “type B”. Subsequently, individuals who inherit A and B together will be fine. But those who inherit either A without B or vice versa will die.

Computer models show that modifying just three per cent of the population is enough to spread the genes. “Such a low threshold was a bit of a shock to us,” explains Davis.

Selection pressure

Hybrid offspring from matings between engineered males and wild females are fine because they have one A and one B. But things get interesting when the hybrids start mating. While all the offspring from hybrid/engineered crosses are fine, some of the offspring from the hybrid/wild crosses die because they have an A or a B in isolation.

This creates a selection pressure which drives the genes through the population, as the offspring of wild flies die more often than those of engineered or hybrid flies.

You could exploit this “drive mechanism” to tackle malaria. Other genes, that either kill the malaria parasite or make the flies susceptible to insecticides, can be tagged onto the modified genes, and will also spread through the population.

While no one has tried this out on real flies, Davis thinks it could be the first practical way to modify a whole population.